1. Field of the Invention
This invention relates in general to a receiver for demodulating the carrier signal in a telephone communications system and in particular to an oscillator for generating the reference signals used to demodulate the carrier signal.
2. Description of the Prior Art
The telephone system in the United States and throughout most of the world is controlled by a plurality of switching centers. A local switching center which is the lowest class switching center is typically referred to as an end central office and is directly connected to a selected number of telephones and other subscriber equipment. The end central offices are connected to toll switching centers which in turn are connected to higher class switching centers.
End central offices in the same area code can be connected together by means of interoffice trunk lines known as EAS (Extended Area Service) trunks. Calls made between subscribers connected to different central offices in the same area code can be routed either through an associated toll center or over an interoffice EAS trunk line if one is available. It is highly desirable to have the end central offices connected by means of trunk lines, as they reduce the overall switching load on the associated toll centers and provide a larger toll-free dialing area for subscribers.
Trunk lines typically utilize a carrier communication system wherein a plurality of communication channels are transmitted and received on a transmission cable at selected carrier frequencies. Each end central office includes suitable modems for modulating the transmit signals and for demodulating the received signals. An interface means is typically provided at each end central office for interfacing the respective modems to the transmission cable.
The characteristics of a typical transmission cable are such that the higher frequency carrier signals are attenuated at a greater rate than the lower frequency carrier signals. To compensate for this greater rate of attenuation, the modems generate the carrier signals at power levels determined by their respective frequencies. The higher frequency carriers are generated at a higher power level than the lower carrier frequencies. This technique of generating the carrier signals at power levels determined by their respective frequencies is typically referred to as presloping the carrier group. When the carrier group signals are all at the same power level, the carrier group is referred to as being flat or having a level slope.
When the distance between central offices becomes relatively great, it becomes desirable to utilize standard repeaters along the transmission cable to maintain the signals above the noise level. In thise case, the central office preslopes the carrier group such that it arrives at the repeater flat. The repeater then amplifies and preslopes the carrier group before transmitting the group to the next repeater. The number of standard repeaters which can be used along a transmission cable is typically limited by the system specifications.
When the transmission cable connecting two central offices is a single pair of lines, the central offices must transmit a channel at one frequency while receiving that channel at another carrier frequency. In a trunk communication system of this type, a technique known as frequency frogging is often utilized at an intermediate repeater for interchanging the frequency allocations of the carrier channels. In this case, each central office transmits the channels at one group of carrier frequencies and receives the channels at a second group of carrier frequencies. A frogging repeater is connected between two central offices and functions to translate the first group of incoming carrier frequencies transmitted by each central office into the second group of outgoing carrier frequencies to be received by each central office. The frogging repeater can also preslope the carrier groups before transmitting the signals to the respective central offices.
One of the problems associated with present trunk communication systems relates to the locations of the repeaters along the transmission cable. The central offices and the standard repeaters typically introduce a predetermined slope in the outgoing carrier group such that the carrier group will be flat after a predetermined length of cable. Since the central office equipment and the standard repeaters are typically designed to receive a relatively flat carrier group, they must be connected together by a cable of the predetermined length. However, since the cable length between two central offices is seldom equal to the predetermined length of a multiple thereof, means must be provided to compensate for the intermediate length cable.
One device used to correct the carrier group slope when the transmission cable is not a multiple of the predetermined length is a frogging repeater manufactured by Anaconda Telecommunications, Anaheim, California. This frogging repeater includes means for connecting additional circuitry, known as line build-out cards, between the incoming carrier cable and the frogging repeater and between the frogging repeater and the outgoing carrier cable. This additional circuitry is selected to simulate the length of cable required to bring the effective cable length up to the predetermined length. Thus, after the incoming signals pass through this additional circuitry, the carrier group will be relatively flat. Similarly, the outgoing group passes through the required number of line build-out cards such that the group will be flat when received by either a standard repeater or a central office. However, a disadvantage of this system is that the frogging repeater must be separately designed for each application.
Another problem associated with present trunk communication systems relates to signaling associated with each communication channel. In addition to providing a path for voice communication, each channel typically provides a path for any signaling associated with that particular channel. The signaling allows the switching equipment at each end central office to communicate and includes such signals as on-hook, off-hook and dialing signals. However, the circuitry required for demodulating the carrier signal and for separating the signaling from the voice signal is typically relatively complex.
The voice communication and signalling information for each channel are recovered from the associated carrier in a receiver. A phase locked loop including a voltage controlled oscillator is contained in the receiver. The phase locked loop allows the receiver to lock onto the frequency of the received carrier signal. The oscillator generates reference signals which are at the same frequency as the carrier signal but in a predetermined phase relationship thereto.
The phase locked loop must not only lock onto the incoming carrier signal but also must maintain lock even if the incoming carrier signal shifts in phase relationship to the reference signal. The time rate of change of phase is frequency. Thus, the oscillator must contain some means which allows its output frequency to vary as a function of the phase difference between the carrier and reference signals.
In receivers which demodulate FM signals, variability in oscillator frequency is achieved by using zener diodes. A reverse biased zener diode has a linear capacitance versus voltage chracteristic. Typically, the capacitance varies from 50 to 100 picofarads over the range of reverse bias voltages. This variation in capacitance is sufficient at the carrier frequencies associated with FM signals to provide variability of oscillator output frequency over a predetermined range. The variation is, however, insufficient at the carrier frequencies typically associated with AM trunk carrier signals to provide the needed range of oscillator output frequency.
At trunk carrier frequencies the typical voltage controlled oscillator utilizes RC circuit elements in order to achieve frequency variation. The frequency variation is achieved by varying the charge current into the capacitor. When the oscillator is used in a receiver which may be subject to a wide variation in temperature for example, 30.degree. F. to 120.degree. F. (-1.degree. C. to 49.degree. C.), oscillators using LC circuit elements will have to be used in place of RC oscillators. LC oscillators have substantially less drift with temperature as compared to RC oscillators and are thus inherently more stable. It is therefore desirable to use LC voltage controlled oscillators at trunk carrier frequencies. It is further desirable to use LC oscillators which allow their output frequency to be simply and easily varied over a desired range in response to differences in phase between the incoming carrier signal and the oscillator output frequency.
A further problem associated with present trunk communication systems relates to the interface means connected between the central office and the transmission cable. The interface means typically includes a carrier group alarm for indicating a system failure. However, most alarm systems typically must be restored manually. Although some alarm systems include a means for automatically restoring the system after recovery, the additional circuitry required is relatively expensive.